For the detection and measurement of combustible gases it is preferable to utilize gas sensors which contain a catalyst and are heated to a specific temperature (for example 500.degree. C.) which will cause the combustible gases to burn catalytically on the surface of the sensor by consuming part of the oxygen present in the measuring gas and to raise the temperature of the sensor. The heat-tone effect generated during the combustion reaction is analyzed and indicated by means of the sensor temperature increase as a measuring signal for the concentration of combustible gas in the air mixture under test. The measuring device for this is generally an active detector and passive compensating sensor in a half arm bridge arrangement. This bridge is fed with either constant current or constant voltage. This type of measuring device operates satisfactorily up to concentrations of 100% LEL (Lower Explosion Limit) of the combustible gas in air. In the case of methane, for example, this limit is 5% by volume. Above this value, a temperature increase in the detector, residual activity in the compensator, oxygen reduction and thermal conductivity changes in the gas mixture all lead increasingly to a non-linear measuring signal with reference to the gas concentration. For gas concentrations in a range above 100% LEL, the bridge output signal does not remain unambiguous, that is, two gas concentrations are found per output voltage value. In order to achieve unambiguous measurements of concentrations over 100% LEL, the measurement bridge used may, for example, be given another arm. This also measures in a bridge arrangement at a reduced operating temperature of, for example, 200.degree. C. only the change in the thermal conductivity of the gas mixture around the sensor. When a specific threshold value for thermal conductivity, which may be in the range of 100% LEL, is reached, the indicator connected to the zero arm of the heat-tone measurement bridge is automatically switched over to full-scale reading via a suitable control element. This gives an indication that the gas sample contains a concentration of combustible gases of over 100% LEL.
This type of measuring device and the method of switching between heat conduction and heat-tone measurement is described in published German patent application DE-OS 16 73 306.
In the case of the measuring device already known, to measure methane in air, for example, one sensor is kept in the heat-tone measuring bridge at an operating temperature of around 500.degree. C. and another sensor is kept in the heat-conduction measurement bridge at an operating temperature of, for example, 200.degree. C. In the measuring range up to 100% LEL the sensor in the heat-tone measurement bridge gives an unambiguous signal. Above the limit of 100% LEL, the measuring instrument in the zero arm of the heat-tone measurement bridge is indeed switched to full-scale reading, but the current or voltage supply of the heat-tone sensor is retained. If the methane content in the air mixture increases further above 100% LEL, the combustion heat on the surface of the catalytic heat-tone sensor increases to such an extent that the consequence may be thermal destruction of the catalyst layer, or at the very least, a substantial reduction in the usability of the sensor for further measurements. There is no point either in switching off the heating current to reduce the operating temperature of the sensor since at these high concentrations catalytic combustion carries on of its own accord and continues either until the gas concentration falls or until the catalyst is contaminated and/or the sensor destroyed.
After switchover of the indicator in the zero arm of the heat-tone measurement bridge to full-scale reading, measurement of the concentration of the actually detectable harmful substance present in the air mixture is no longer possible.
In the case of the measuring device already known, two measurement bridges have to be simultaneously and continuously supplied with electric power; and in particular, both sensors in the measurement bridges have to stay heated to 200.degree. C. or 500.degree. C. in order to guarantee constant measurement readiness. This means increased power consumption because both sensors must be simultaneously maintained at their respective operating temperatures, although only one of them would be required for actual measurement or for fixing the time for switchover of the measuring instrument to full-scale reading. Given the necessary charging time of a measuring instrument, this increased power requirement demands a suitably adapted power supply and in the case of a portable measuring instrument, for example, necessitates carrying around large, heavy and therefore cumbersome batteries.